Selenium rectifier of increased blocking properties



Feh. 28,1956

SELENIUM RECTIFIER OF INCREASED BLOCKING PROPERTIES O. KLEIN Filed May 22, 1952 Selenium with 10 mg '/0 mefal, e. q sodium metal electrode Jeleni iodin e +mefal Selenium iodine semiconductor melpil meld! iodide iodide metal selenid iodine INVENTOR QKLEI N ATT OR EY United States Patent SELENIUM Riemann QETNCREASED BLOCKING rn'ornnrrns 2 Claims. (Cl. 117200) It is well known with selenium rectifiers to add halogen additions to the selenium. The amount of halogen contents determines the properties of the rectifier in both flux and blocking direction. The resistance of the selenium is considerably reduced by the halogen contents. This, however, is also desired for the great mass of the semiconducting layer. At the surface of the selenium a blocking layer is formed. This blocking layer should be rather highly resistive in order to achieve a high blocking voltage and should, therefore, contain as little as possible or no halogen at all. It is therefore known since a rather long time, to build up selenium rectifiers in a way, that a halogen-poor or halogen-free selenium layer is vaporized upon a halogen-containing selenium layer.

Owing to diffusion, which is still sponsored by high temperatures, the initially established condition of the halogen distribution cannot be maintained on the long run. Moreover, the halogen migrates gradually into the actual blocking layer after having been transported before, amongst others by the electrical formation, out of the actual blocking layer. Owing to these processes of diffusion the properties of the rectifiers will be deteriorated in the course of time.

Furthemore it is known that additions of metal make the selenium highly resistive. Especially such metals are suitable which, when being melted, rather form selenides than oxides. These are for example, alkaline or alkaline earth metals, both of which are included by the term alkaline metals. If now in the selenium, additionsof halogen are existing as well as additions of metal, then the transition from the conductivity-increasing to the conductivity-decreasing influence will be characterized perhaps by the stoechiometric relation of metal and halogen. Decisive for this stoechiometric relation is the respective stable metal halogenide, such as sodium-iodide. Therefore, the fact is to be taken into account that, whenever the metal in the blocking layer is available in surplus, the blocking layer zone will remain highly resistive also at the diffusing into, of halogen from the semi-conducting layer.

According to the invention, however, the selenium rectifier will be built up in such a manner, that on to a selenium layer having high contents of halogen amounting to a strength up to 100 and halogen contents amounting up to 100 mg. a very thin halogen-free selenium layer is deposited, which is provided with conductivitydecreasing metal additions. Therefore, additions of alkaline metals may be added to the selenium of this second thin layer. Both layers may be deposited according to the known methods. They both may be pressed on or evaporated. It is of advantage to press on the first, halogenated layer, in the usual manner on air, because a high content of halogen is guaranteed in this way. The second thin halogen-free selenium layer is con veniently being evaporated.

A better understanding of the idea of this invention 2 may be had from the description given below and made with reference to the copending drawing", in which:

Fig. 1 shows the conductivity of the selenium in' deendency of d'iifer'ent contents of sodium and iodine, and

Fig. 2 shows the most favourable dosing of iodine and metal in the blocking layer z'one. The examination of the conductivity behaviour offselenium has resulted in that the adding of metals tohaloeriated selenium'leads' to quick decrease of the conductivity in that particular moment, in wt ichtiie metal is available in the amount corresponding ap roximately to the chemically stable compound of the metal-halogenide. Certain metals, such as alkaline metals (sodium) show this effect in a specially distinct manner, as may be seen from Fig. 1. At about 50% each, of sodium and iodine, the curvature of conductivity a shows a severe drop.

According to certain presumptions, the concentration of disturbing points will be reduced by saturating the iodine with metal for obtaining a metal-iodide, thus diminishing the conductivity. Metals which already form oxides when being smelted on air, however, are of no influence upon the conductibility.

At a melting down, therefore, it is required that the added metal first of all must chemically combine the iodine, and the resulting surplus must then be available in the shape of selenide and not as oxide. When being melted down in an iodine-free selenium, the metal has to form selenides and may not oxidize prior thereto. If this is the case then the layer, which is deposited on the halogenated layer, is regarded as being highly resistive and, therefore, suitable as a blocking layer. The blocking layer zone is said to consist of a metal-selenidecontaining selenium, whereby the equilibrium of reaction is always shifted towards the right in the following order of succession: metal-oxide, metal-selenide, metal-iodide. If halogen migrates into the blocking layer, for instance, by difiusion or electrolysis, then a part of the excessive metal-selenide changes into metal-iodide, bufiering therewith with the halogen. The blocking layer, however, remains halogen-poor, i. e. highly resistive.

For the purpose of achieving layers of high resistance, it is suggested by the present invention to carry out a laminated building up by means of selenium layers containing halogen and metal respectively. A distribution of the added additions in the blocking layer will then be performed by diiferent diifusing processes, as represented in Fig. 2 of the drawing. A very highly resistive layer, providing the rectifier with excellent properties, will then be formed approximately near the point of intersection of the two graphic curves representing the metal and iodine concentration.

As mentioned already before, after this application of the counter electrode, one will have to reckon with processes of difiusion and with electrolysis, owing to which the halogen gradually will enter the actual blocking layer, whereby the blocking effect will be aiiected. In order to support the effect of the metal, existing in the blocking layer, it will be convenient to add recrystallization-retarding substances such as arsenic or phosphorous and/ or dilfusion-retarding additions, such as neutral metal oxides (A1203) to the selenium. It is easy to perceive, that the recrystallization-retarding additions should substantially be existent in the blocking layer, whilst the diffusion-retarding additions should perform their effect in the halogen-containing semi-conducting layer. The different components may be added singly or in mixtures. The added quantities may amount up to mg. percent for each component.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

l. A selenium rectifier comprising a base electrode, a halogen-containing selenium layer on said base electrode, and a second selenium layer substantially free of halogen, containing as an additive an alkaline metal and a crystaJlization-retarding substance taken from the class cons'isting of arsenic and phosphorous.

2. A selenium rectifier according to claim 1 wherein said selenium contains a diffusion-retarding addition of aluminum oxide.

References Cited in the file of this patent UNITED STATES PATENTS Hewlett Nov. 16, 1943 Smith Nov. 16, 1948 FOREIGN PATENTS Great Britain Mar. 25, 1938 Great Britain Sept. 29, 1945 France Dec. 6, 1948 France Feb. 21, 1949 

1. A SELENIUM RECTIFIER COMPRISING A BASE ELECTRODE, A HALOGEN-CONTAINING SELENIUM LAYER ON SAID BASE ELECTRODE, AND A SECOND SELENIUM LAYER SUBSTANTIALLY FREE OF HALOGEN CONTAINING AS AN ADDITIVE AN ALKALINE METAL AND A CRYSTALLIZATION-RETARDING SUBSTANCE TAKEN FROM THE CLASS CONSISTING OF ARSENIC AND PHOSPHOROUS. 